Field of the Invention
The present invention relates to a controller for a machine tool, and more particularly, to a controller for a machine tool configured to perform efficient warm-up control based on a thermal displacement state at the point in time when thermal displacement is saturated.
Description of the Related Art
In a machine tool, a feed screw and a spindle are driven by a motor, so that they are expanded to inevitably change the machine position by heating of the motor, frictional heat due to rotation of a bearing, and frictional heat from a contact portion between a ball screw and a ball nut. Thus, the relative positions of a tool and a workpiece to be machined are shifted. This change of the machine position due to the heating causes a problem in high-precision machining.
The displacement of the machine position by heat can be removed by using the following methods. One of these methods is a technique in which a command position is compensated based on a detected displacement or temperature measured by a displacement or temperature sensor (thermal displacement compensation). In another method, initial tension is applied to a feed screw to avoid the influence of thermal expansion. A third method is a warm-up method in which a machine is operated to stabilize accuracy before machining a workpiece.
In warm-up operation, the spindle is rotated at a speed used for actual workpiece machining or idling is repeated to run a machining program without a workpiece mounted in the machine. The machining accuracy can be stabilized by performing idling until the thermal displacement of the machine tool is stabilized. However, the time and details of the idling are determined based on the experience and intuition of a skilled operator, so that wasteful warm-up time may possibly occur. To overcome this, a method for determining the timing for ending warm-up operation and a method for efficiently performing warm-up operation are proposed in, for example, Japanese Patent Applications Laid-Open Nos. 07-124846, 2004-261934, and 08-215981.
A machine tool described in Japanese Patent Application Laid-Open No. 07-124846 is constructed based on the observation that the motion of a hydraulic actuator is less smooth than usual in the first morning operation and inevitably fails to pass various abnormality tests during warm-up operation. When a warm-up mode is selected in this machine tool, the position tolerance of a hydraulic servo system and the set value of position arrival time are increased so that some predetermined operations can be omitted by ignoring detection signals from a workpiece detection switch. Thus, the machine tool can smoothly perform warm-up operation without a useless interruption of operation.
Although the warm-up time is reduced by obviating a useless operation stop according to the technique described above, however, a machining program for idling is one designed for use in actual machining. Therefore, the warm-up operation involves many wasteful operations, such as a dwelling operation commanded during machining, low-speed machining operation, and tool change operation, and entails extra time for that.
A machine tool described in Japanese Patent Application Laid-Open No. 2004-261934 can reduce the warm-up time by such control as to prevent wasteful warm-up operation. In this control, the cutting-edge position of a tool is detected during warm-up operation of a spindle, and it is determined that there is no need of warm-up operation if the change of the tool cutting-edge position is within a preset tolerance.
Although the timing for ending the warm-up operation can be determined according to the technique described above, the reduction of the warm-up time is limited, since the efficiency of the warm-up operation is invariable. Further, the object to be the warmed up is limited to the spindle, and there is no description of warm-up operation of a feed screw in the patent document.
A machine tool described in Japanese Patent Application Laid-Open No. 08-215981 can reduce the warm-up time in the following manner. A spindle is rotated at a speed used for actual machining and a thermal displacement amount at the time of saturation of thermal displacement is measured in advance. In actual warm-up operation, the spindle is rotated at a higher speed for preheating so that a thermal displacement amount equal to the measured value is reached. Thereafter, the rotational speed for the actual machining is restored, and machining is started with relative positions compensated by an amount corresponding to the generated thermal displacement amount. In this way, a high machining accuracy equivalent to that for the case where the stability of the thermal displacement is awaited before starting the machining can be obtained, and at the same time, the warm-up time can be reduced.
According to the technique described above, the timing for ending the warm-up operation is determined by comparing a thermal displacement amount in a case where the thermal displacement is saturated with a current thermal displacement amount. Since a thermal displacement amount of a spindle involves deformations of a spindle mount and a column and the thermal displacement does not simply increase but complicatedly changes, however, the warm-up operation may sometimes be ended in an untimely manner. Further, the above patent document describes only the warm-up operation at a certain rotational speed for preheating and does not describe efficient warm-up operation taking into consideration acceleration and deceleration of the spindle. Furthermore, the object to be warmed up is limited to the spindle, and there is no description of warm-up operation of a feed screw in that patent document.